1. Field of the Invention
The invention relates to a process for operating a Coriolis mass flow rate measurement device which has at least one measurement tube, the measurement tube being excited into vibrations with a predetermined excitation frequency and a predetermined excitation phase.
2. Description of Related Art
Coriolis mass flow rate measurement devices (CMDs) generally have a single measurement tube or a plurality of measurement tubes through which a medium flows, with a mass rate of flow which is to be determined. Thus, for example, there are Coriolis mass flow rate measurement devices with a single straight measurement tube and Coriolis mass flow rate measurement devices with two measurement tubes which run curved.
It is common to these Coriolis mass flow rate measurement devices that the measurement tubes through which a medium flows form a mechanical vibration system which is influenced by the flowing medium. Thus, the density of the medium changes the resonant frequency of the vibration system, while the mass rate of flow of the medium changes the vibration form. As a result, in addition to the mass rate of flow, among other things, also the density of the flowing medium can be determined.
Coriolis mass flow rate measurement devices are characterized by high measurement precision. Thus, measurement of the mass rate of flow with a precision of less than 0.1% is possible. Moreover, with Coriolis mass flow rate measurement devices, in addition to the mass rate of flow, other values can be determined which are derived, in part, from primary measurement values. Examples of these derived parameters of the medium flowing through the measurement tube are the volumetric flow rate, the mass or volumetric amount and the concentration of the flowing medium. Thus, Coriolis mass flow rate measurement devices are multivariable measurement devices which are often used not only for a primary measurement tasks, but in addition can deliver important secondary diagnosis and quality information about a process.
The problem in Coriolis mass flow rate measurement devices is multiphase flows, such as two-phase flows. Examples of multiphase flows, specifically of a two-phase flow, are gas bubbles in a liquid which can be caused, for example, by cavitation in valves or pumps or intake of air at leaks. Furthermore, one example of a two-phase flow is a system of solids in a liquid, for example, caused by crystallization or sudden detachment of deposits in a pipeline system through which the medium is flowing. Finally, there is the example of two-phase flows in mixtures of insoluble liquids, therefore emulsions, which can be caused, for example, by changing the medium which is flowing through the pipeline system.
If, during operation of a Coriolis mass flow rate measurement device, a multiphase flow occurs or the ratio of the phases of the multiphase flow changes, generally, also the resonant frequency of the measurement tube through which the medium is flowing changes. Since the measurement tube is typically excited at the resonant frequency, thus the vibration excitation of the measurement tube must be readjusted. In conventional processes, this is often associated with the loss of the operating point, i.e., the frequency and phase of the vibration excitation must be re-determined so that, essentially, the Coriolis mass flow rate measurement device must be restarted. However, during the restart phase, it is not possible to measure the mass rate of flow.